Mobile communication devices, such as cellular telephones, PDAs, handsets, and laptop computers, require antennas for wireless communication and previously used multiple antennas for operation at various frequency bands. Recent wireless devices, however, use a single antenna to operate in multiple frequency bands. One such frequency range increasing in popularity is the ISM band (2.4 GHz), which covers frequencies between 2.4-2.4835 GHz in the United States with some variations in other countries. Different protocols are used to transmit and receive signals in this band: the Bluetooth Standard published by the Bluetooth Special Interest Group and the IEEE Standard 802.11b published by the Institute of Electrical and Electronic Engineers. The UNII (Unlicensed National Information Infrastructure) band covering the 5-6 GHz range is another frequency band that has been recently allocated (specifically, a 200 MHz block at 5.15 MHz to 5.35 MHz and a 100 MHz block at 5.725 MHz to 5.825 MHz) to alleviate some of the problems that plague the 2.4 GHz band, e.g. saturation from wireless phones, microwave ovens, and other emerging technologies. The UNII band uses IEEE Standard 802.11a, which supports data rates of up to 54 Mbps and is faster than the 802.11b standard, which supports data rates of up to 11 Mbps. In addition, unlike the 802.11b standard, the 802.11a standard departs from spread-spectrum technology, instead using a frequency division multiplexing scheme that""s intended to be friendlier to office environments. Of course, there are many other frequency bands over which wireless devices may operate, including the 800 MHz, GSM and PCS, GSM and DCS, or GPS L1 and L2 bands.
As one example of conventional antennas that operate in multiple frequency bands, including the 2.4 GHz range, SkyCross has triband antennas (antennas operating in three frequency ranges) that range in size from 20xc3x9718xc3x973 mm to 22.3xc3x9714.9xc3x976.2 mm. The smallest antenna has an efficiency of better than 60% but a poor Voltage Standing Wave Ratio (VSWR) of less than 3:1 (the larger antenna has an improved VSWR of 2:1 but an unreported efficiency). Other manufacturers include Ethertronics, having an antenna only matched to xe2x88x926 dB across the upper band (with a peak efficiency of 75% based on the shown return loss plot), and Tyco Electronics, having a circular antenna of 16 mm diameter and 6 mm height with a better than 2.5:1 VSWR but again, unreported efficiency.
Ample room remains for improvement in multiple areas of interest for these antennas for the designer, manufacturer and ultimately consumer with the ever-increasing demand for smaller and lighter (as well as cheaper) consumer electronics. These areas include not only the efficiency and overall performance, but also the cost, size and weight of the antenna. Of course, other conventional antennas used in other mobile communication devices face similar problems; the antenna performance is inherently linked to the size of the antenna as there is a fundamental limit on the efficiency and bandwidth that can be achieved based on the total volume of the antenna. In consequence, manufacturers of consumer electronics, who have little room in their products for antennas given the size and cost pressures, have conflicting interests to improve the device performance.
In addition to the size/performance tradeoff noted above, other problems occur when attempting to design antennas using frequency bands that are separated by large amounts, for example an octave or more apart. One such problem is the limiting of the higher frequency bandwidth due to reactive loading by the lower resonance. Adding to this, the antennas must be designed for low cost manufacturing as well as contain low cost materials to be cost effective for use in consumer electronic devices. This has led to the incorporation of the antenna within the package or case for reasons of durability and size.
Such wireless devices typically pack a substantial amount of circuitry in a very small package. The circuitry may include a logic circuit board and a radio frequency (RF) circuit board. The printed circuit board (PCB) can be considered an RF ground to the antenna, which is ideally contained in the case with the circuitry. A preferred antenna for use in these wireless devices would be one that can be placed extremely close to such a ground plane and still operate efficiently without adverse effects such as frequency detuning, reduced bandwidth, or compromised efficiency.
Various antennas have been developed to provide capability in at least one of the 2.4 and 5-6 GHz ranges. These include Planar Inverted-F Antennas (PIFAs), types of shorted patches, and various derivatives, which may contain meander lines. However, the need to integrate a single, compact, antenna structure that responds (i.e. has resonant frequencies) in both the 2.4 and 5-6 GHz ranges remains. Thus, to date, none of the above antennas satisfy present design goals, in which efficient, compact, low profile, light weight and cost effective antennas are desired.
To achieve the above objectives, in addition to other objectives mentioned herein, combination PIFA/reverse-fed planar inverted F-antennas (RFPIFA) having frequency response in multiple frequency ranges are disclosed in various embodiments below.
In one embodiment, the multiband antenna comprises a PIFA having a first resonant frequency and a RFPIFA surrounding the PIFA on two sides and having a second resonant frequency lower than the first resonant frequency. In another embodiment, the multiband antenna the RFPIFA surrounds the PIFA on three sides.
In a third embodiment, the PIFA and RFPIFA have first and second resonant frequencies, respectively, (with the RFPIFA resonant frequency lower than the PIFA resonant frequency) as well as being integrally formed from a single piece of conductive material and attached at one end such that dimensions of the multiband antenna are defined substantially by the RFPIFA.
Any of the embodiments may contain the elements below.
The multiband antenna may comprise an out-of-plane matching stub to impedance match the multiband antenna with external elements. This stub may extend from the feed line. The length and width of the stub as well as distance between the stub and the ground plane (i.e. the height of the stub) is chosen to optimize the impedance match. Similarly, an antenna element that has a third resonant frequency higher than the first resonant frequency may be disposed perpendicular to the ground plane.
The conductive material that forms the PIFA and RFPIFA may be separated from a ground plane by two layers having an effective permittivity of about 1 to about 1.7. The PIFA/RFPIFA may be disposed on an undercarriage, which is in turn supported by legs. The thickness of the undercarriage is about 0.3 to 1.0 mm and the overall thickness of the antenna is about 2 mm to 4 mm. The legs contact the ground plane such that the undercarriage is mounted on a printed circuit board (PCB) and the PIFA and RFPIFA are mounted over components mounted on the PCB. The legs may be plastic with metalized contacts positioned on the PCB for solder reflow connection. The multiband antenna may be mounted at an edge of the PCB.
The resonant frequencies of the PIFA and RFPIFA may be adjustable by removal of a portion of the PIFA or RFPIFA or addition of inductance at discrete locations including formation of a narrow inductive transmission line in the RFPIFA or between the PIFA and RFPIFA.
The multiband antenna may be devoid of dielectric loading and meander lines or may have one or more meanderlines having the same shape. A narrow inductive transmission line may be disposed between the meanderlines.
The largest dimension of the RFPIFA is at most {fraction (1/10)} of the second resonant frequency without dielectric loading. The resonant frequency of the PIFA may be 5 to 6 GHz while that of the RFPIFA about 2.4 GHz.
The multiband antenna may be relatively insensitive to proximity effects and to changes in ground plane size and component layout on a PCB on which the multiband antenna is mounted.
In a fourth embodiment, a method for multiband reception of an antenna comprises communicating in first and second resonant frequencies via a PIFA and RFPIFA, respectively, (with the RFPIFA resonant frequency lower than the PIFA resonant frequency) and limiting an area of the PIFA and RFPIFA such that dimensions of the antenna are defined substantially by the RFPIFA.
In a fifth embodiment, a method for multiband reception of an antenna comprises communicating in first and second resonant frequencies via a PIFA and RFPIFA, respectively, (with the RFPIFA resonant frequency lower than the PIFA resonant frequency) and adjusting one of the resonant frequencies by one of removing a portion of the PIFA or RFPIFA or addition of inductance at discrete locations including forming a narrow inductive transmission line in the RFPIFA or between the PIFA and RFPIFA.